1. Field of the Invention
This invention relates to a method and apparatus for converting LPG boiling range paraffins to heavier hydrocarbons, such as gasoline range or distillate range fuels. In particular, it relates to methods and apparatus for doing so, which integrate the operation of catalytic or thermal dehydrogenation of a paraffinic feedstock to produce olefins with the operation of a catalytic reactor zone to convert olefins to heavier hydrocarbons, and which further include downstream separation of heavier hydrocarbons to optimize product selectivity and minimize the heat requirement. The method and apparatus may also include a fluid bed dehydrogenation zone to dehydrogenate paraffins to olefins.
2. Discussion of the Prior Art
The conversion of light paraffins, such as propane and butane, to mono-olefins, such as propylene and butylene, has been accomplished by thermal or catalytic dehydrogenation. A general discussion of thermal dehydrogenation (i.e., steam cracking) is presented in Encyclopedia of Chemical Technology, Ed. by Kirk and Othmer, Vol. 19, 1982, Third Ed., pp. 232-235. Various processes for catalytic dehydrogenation are available in the prior art. These processes include the Houdry Catofin process by Air Products and Chemicals Inc., Allentown, PA, the Oleflex process by UOP, Des Plaines, IL, and a process disclosed by U.S. Pat. No. 4,191,846. The Houdry Catofin process, described in a magazine article, "Dehydrogenation Links LPG to More Octanes", Gussow et al, Oil and Gas Journal, Dec. 8, 1980, involves a fixed bed, multi-reactor catalytic process for conversion of paraffins to olefins. Typically, the Houdry Catofin process runs at low pressures of 5-30 inches of mercury absolute, and high temperatures with hot reactor effluent at 550.degree.-650.degree. C. Dehydrogenation is an endothermic reaction, so it normally requires a furnace to heat a feedstream prior to feeding the feedstream into the reactors. The Oleflex process disclosed in an article "C.sub.2 /C.sub.5 Dehydrogenation Updated", Verrow et al, Hydrocarbon Processing, April 1982, uses stacked catalytic reactors with hydrogen recycle. U.S. Pat. No. 4,191,846 teaches the use of Group VIII metal containing catalysts to promote catalytic dehydrogenation of paraffins to olefins.
Recent developments in zeolite catalysts and hydrocarbon conversion methods and apparatus have created interest in utilizing olefinic feedstocks for producing heavier hydrocarbons, such as C.sub.5.sup.+ gasoline or distillate. These developments have contributed to the development of the Mobil olefins to gasoline/distillate (MOGD) method and apparatus.
In MOGD, olefins are catalytically converted to heavier hydrocarbons by catalytic oligomerization using an acid crystalline zeolite, such as a ZSM-5 type catalyst. Process conditions can be varied to favor the formation of either gasoline or distillate range products. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in combination with paraffinic components, into heavier hydrocarbons over a crystalline zeolite catalyst. Garwood et al have contributed improved processing techniques to the MOGD system, as disclosed in U.S. Pat. Nos. 4,150,062; 4,211,640; and 4,227,992. Marsh et al, as disclosed in U.S. Pat. No 4,456,781, provided improved processing techniques for the MOGD system. U.S. Pat. No. 3,760,024 to Cattanach discloses contacting olefins with ZSM-5 type zeolite catalyst to form aromatics.
The conversion of olefins in a MOGD system may occur in a gasoline mode or a distillate mode. In the gasoline mode, the olefins are catalytically oligomerized at temperature ranging from 400.degree.-800.degree. F. and pressure ranging from 10-1000 psia. To avoid excessive temperatures in the exothermic reactor, the olefinic feed may be diluted. In the gasoline mode, the diluent may comprise light hydrocarbons, such as C.sub.3 -C.sub.4, from the feedstock and/or recycled from debutanized product. In the distillate mode, olefins are catalytically oligomerized at temperature ranging from 300.degree.-600.degree. F. and pressure ranging from 400-1000 psig. In the distillate mode operation, olefinic gasoline may be recycled and further oligomerized, as disclosed in U.S. Pat. No. 4,211,640 (Garwood et al).
Olefinic feedstocks may be obtained from various sources, including from fossil fuel processing streams, such as gas separation units, from cracking of C.sub.2.sup.30 hydrocarbons, from coal by-products and from various synthetic fuels processing streams. U.S. Pat. No. 4,100,218 (Chen et al) teaches thermal cracking of ethane to ethylene, with subsequent conversion of ethylene to LPG and gasoline over a ZSM-5 type zeolite catalyst.
Although gasoline and distillate can be produced from propane and butane by the prior art using dehydrogenation integrated with MOGD, there are several problems with integrating these processes. For example, U.S. Pat. No. 4,413,153 (Garwood et al) discloses a system which catalytically (or thermally) dehydrogenates the paraffins to olefins, and then reacts the olefins by catalytic oligomerization (MOGD) to distillate range material. Catalytic oligomerization in the distillate mode is a high (preferably greater than 600 psig) pressure process, whereas dehydrogenation is favored by lower (less than 25 psig) pressure.
Typically, H.sub.2 separation of the dehydrogenation effluent stream could be accomplished by feeding the stream to an absorber. In the absorber, the dehydrogenation effluent stream contacts with a lean oil stream, which absorbs C.sub.3.sup.+ material from the dehydrogenation effluent stream to form a C.sub.3.sup.+ rich stream. Meanwhile, H.sub.2 and light gases exit the absorber in a C.sub.2.sup.- rich stream. The C.sub.3.sup.+ rich stream is sent to a debutanizer to form a debutanizer overhead stream and a debutanizer bottoms stream.
The debutanizer bottoms stream forms the lean oil stream which is recycled to the absorber. Thus, the absorber has a lean oil stream circuit which is independent of downstream processing. Typically, the debutanizer overhead stream comprising C.sub.3 's and C.sub.4 's would pass to an oligomerization reactor zone, where, in order to increase the distillate yield from the oligomerization reactor zone, it is combined with a recycle gasoline stream comprising C.sub.5.sup.+ olefins recovered from an oligomerization reactor zone effluent.
It is difficult to efficiently separate the effluents from the oligomerization reactor zone to provide a recycle gasoline stream to the oligomerization reactor zone as well as a gasoline product stream. One such separation system, disclosed in U.S. Pat. No. 4,456,781 to Marsh et al, separates oligomerization reactor effluent in a debutanizer to form a C.sub.5.sup.+ bottoms stream, which is fed to a splitter where it vaporizes all the gasoline to form a gasoline product overhead stream and a distillate product bottoms stream. It is inefficient because the gasoline is vaporized only to be subsequently cooled to a liquid. It would be more efficient to recover gasoline for recycle as a liquid prior to the splitter to conserve the energy required to vaporize the gasoline recycle.
Another problem with integration of dehydrogenation with oligomerization is the dehydrogenation of LPG paraffins to olefins is thermodynamically unfavored and very endothermic. Supplying heat for the process leads to a significant part of the process costs for current technology. U.S. Pat. No. 4,032,432 combines the operation of a fluid catalytic cracking (FCC) unit with a catalytic oligomerization zone to send heat from the FCC unit to the catalytic oligomerization zone. However, catalytic dehydrogenation of LPG range paraffins has not been combined with an FCC unit to recover heat.